The present invention is generally related to decimation circuits used, for example, in oversampled delta-sigma analog-to-digital converters, and, in particular, to a decimation circuit and method for filtering quantized electrical signals while providing a substantially uniform magnitude and a substantially linear phase response at least over a desired passband range.
While data acquisition systems for generating digital data for the purposes of computation may receive analog input signals from a plurality of sensors, the analog signals must be digitized or quantized before they can be used by a computer as a basis for supporting computations. It is desirable to include respective analog-to-digital converters within the confines of an inexpensive single monolithic integrated circuit. Such data acquisition circuitry can be constructed using metal-oxide-semiconductor (MOS) integrated circuit technology and is suited for applications such as power metering and internal-combustion engine control.
Oversampling analog-to-digital converters of delta-sigma type are particularly economical of digital hardware. The use of such converter introduces the need for sinc.sup.k decimation filters, in which the kernel is a sampled-data representation of a suitable time-domain response, to achieve sufficient selectivity against harmonic components of the sinusoid being filtered. For example, for k=1 the time domain response corresponds to a rectangular time response, while for k=2, the time domain response corresponds to a triangular time response. In each case, the magnitude response of such sinc.sup.k filters generally introduces considerable attenuation over the high-end of a desired passband range. Although magnitude correctors have been suggested, in general such suggested magnitude correctors typically exhibit an undesirable nonlinear phase response over the bandpass range of interest. For example, L. B. Jackson, "Digital Filters and Signal Processing", 1986, available from Kluwer Academic Publishers, discusses in pages 76 and 77 various magnitude correctors, however, no suggestion is made of how to advantageously provide both a substantially uniform magnitude and a substantially linear phase response over the desired bandpass range. Thus it is desirable to provide a magnitude corrector capable of operating in a manner consistent with providing a substantially linear phase response over the passband range of interest.